WO1994002207A9

WO1994002207A9 - Ambient air catalytic purifier and corresponding catalysts

Info

Publication number: WO1994002207A9
Application number: PCT/FR1993/000693
Authority: WO
Filing date: 1993-07-06
Publication date: 1994-05-26

Abstract

Ambient air purifier which eliminates toxic compounds and odours. The association of different constituent parts provides clean air for use inside buildings and also renews air in the vicinity of roads or polluted areas. The air purifier is characterized in that it can be installed near the persons using it and also near very busy and/or polluted roads.

Description

CATALYTIC APPARATUS FOR PURIFYING AMBIENT AIR AND CORRESPONDING CATALYSTS

The present invention relates to an apparatus for purifying air for human respiration, this purification being achieved by eliminating toxic compounds and odors. The combination of the various parts of it makes it possible to produce clean air that can be used inside the buildings, but it can also be used to renew the air in polluted streets or neighborhoods.

This device has the characteristic of being able to be installed close to the people who use it and it is also designed to be installed in the streets very transited and / or very polluted. STATE OF THE PRIOR ART

There are domestic or industrial air purification units based on one or more filtration actions with filters of fibers, fibers or activated carbon and, in some cases, having a complementary action of generation of negative ions. These filters are effective at retaining suspended particles or aerosols but their usefulness is reduced when it comes to the gaseous pollutants present in the ambient air of big cities (carbon monoxide, hydrocarbons, ozone, peroxyacetyl nitrate, benzopyrene, etc ...).

There are, on the other hand, catalytic systems (converters used in industry to purify industrial gaseous effluents) which are characterized by the use of a relatively high temperature and the need to preheat the gases before the reaction and to cool partially after the reaction. These problems are relatively easy to solve on an industrial scale, by the use of heat exchangers tubes or plates operating in steady state against the current, or by the implementation of cyclic regenerators which store and restore the heat. However, on an industrial scale, we have never sought to simultaneously reduce the volume of heat exchangers, the energy cost and the investment cost of energy. these devices as well as their sound level.

These simultaneous reductions are a critical factor for a catalytic system to have an acceptable price for applications that are more directly relevant to people who need purified air, that is, for clean-up of premises and streets. , the unit cost of the treated m3 must be very low.

It is an object of this invention that, by the combination of different means, a substantial reduction in the volume of the apparatus, its energy consumption and investment cost can be achieved simultaneously and, consequently, obtain a low level system. price that can be installed close to people (house, automobile, school, trade,

6 υC _{• • •)} m As we will see in what follows, this important cost reductions, as a result of the present invention not only to design a household appliance for low prices, but also capable of purifying device large airflows in city streets. In other words, the apparatuses according to the invention can purify air flows of between 20 m 3 / hour and 20000 m 3 / hour and the principles of their construction are identical in these two extreme cases.

Its use is specifically for the polluted air of a building or a street, that is to say containing pollutant contents of the order of a few parts per million. Another characteristic is that it is always a question of treating cold air, that is to say introduced into the apparatus at ambient temperature. Prior to disclosure of this invention, a catalytic method had never been used to purify ambient and low temperature air. The apparatus is designed primarily for use with commercially available catalysts; these catalysts, based on Palladium, reduce the pollutant content (ozone, reactive hydrocarbons, carbon monoxide, peroxyacetyl nitrate) and they operate at a temperature between 70 and 300 degrees Celsius. It is estimated that with

SUBSTITUTE SHEET these catalysts, the ozone can be destroyed at 70 ° C and that the carbon monoxide and the reactive hydrocarbons can be destroyed at temperatures of the order of 240 ° C. The saturated hydrocarbons are burned at temperatures higher than 300 ° C but in the case of the decontamination of the ambient air, it is not necessary to eliminate these hydrocarbons which are not toxic and are not precursors of compounds of the ozone type.

As will become apparent in the following, new commercially unavailable catalysts, which we have studied, allow for greater efficiency of the reactor at a lower cost of the catalyst.

The characteristic details of this catalytic purifier are clearly indicated in the description which follows and in the accompanying drawing for purposes of illustration and using the same reference signs.

DESCRIPTION OF THE DRAWING OF THE INVENTION Figure 1 shows the schematic diagram of the purifier. DETAILED DESCRIPTION OF THE INVENTION

A fan (1), provided with a filter (2) for removing dust, suspended particles, etc., blows the air to be purified from the street or the building itself, in a system of pipes, tees and elbows, equipped with two or more solenoid valves (3) which, by action of a cyclic controller (11) allow to introduce this air to one or the other of the two inputs of the reactor. In the position indicated in the figure, the air enters (4) in the reactor and exits at (5). In the second half of the cycle, after the inversion of the solenoid valves, the air enters (5) and leaves in (4).

The reactor consists mainly of three parts:

The element (6) is the catalyst of the type mentioned in the foregoing or, preferably, of the type mentioned in the following.

The sections (7 and 8), filled with a non-catalytic solid, act as exchangers, or more exactly,

SUBSTITUTE SHEET heat accumulators. When air at room temperature enters (4), it preheats in section (7) to a temperature close to that of the reactor (about 220 ° C). At the exit of the section (6), it enters the section (8) in which it gives up its sensitive heat which remains stored (accumulated) in the lining.

After inversion of the solenoid valves, the section (8) serves as preheater of the air and the section (7) cooler.

It is obvious that, after an initial phase, a system such as the one we are describing could work indefinitely provided that the air is cooled to a temperature equal to its inlet temperature.

To achieve this, it would be necessary that the exchange sections have a very great length, which obviously is not economical (high volume and high pressure drop). The outlet temperature in (5) is therefore necessarily higher than the inlet temperature in (4). Therefore, the apparatus according to the invention is at the same time a purifier and an air heater and even with a very efficient heat exchange system such as that claimed in the invention thermal energy must be provided to compensate for the inefficiency of the heat exchanger.

This input is in the form of electrical energy and FIG. 1 indicates the method recommended in the invention for introducing this electrical energy into the system. Note that the operation of the device is completely electric (fan, solenoid valves and heating energy). An electrical resistor (9) is placed at half of the catalyst bed to provide the heat necessary for the operation. This resistor (9), connected to the main switch of the device (common to the fan motor, the programmer and the resistor) is kept active during the preheating time and during the actual operation (speed pseudo-stationary) and this with the same intensity of electric current.

It is obvious that the apparatus must be "symmetrical": the two exchange sections (7 and 8), the tubes (4 and 5) and

SUBSTITUTE SHEET the valves (3) are identical.

After pressing the main switch that simultaneously switches on the fan (1), the solenoid reversing programmer (11) and the resistor (9), the catalyst temperatures (6) and the packings increase. to stabilize, after a certain number of cycles, at the temperatures chosen during the design of the apparatus.

The essential property of the apparatus according to the invention is to minimize the supply of energy to the resistor (9) and the fan (1) without prejudice to purification and without increasing the investment cost. To achieve this result, it was found that the heat exchange system (Sections 7 and 8) had to meet the following conditions: a) Increase the efficiency of the heat exchange through the use of a lining large surface area (square meters of surface per cubic meter of lining). This can only be achieved by the use of small diameter metal wires of relatively stainless material. The efficiency of a packing is defined in scientific terms by the NTU (Number of Transfer Units), ie Number of Transfer Units, per unit length. For the same length of the lining, the heating and cooling will be even greater than the value of the NTU is large.

With the metal fibers, of very small diameter, this efficiency can be increased and thus reduce the length of packing required but this results in a loss of pressure in the lining which is too high, which is not compatible with the mentioned criteria, relating to to an economical apparatus and low acoustic level. b) It has been found that the best compromise between volume, thermal efficiency and pressure drop is achieved by means of relatively high volume porosity packings (75 to 95

%) made of threads of a diameter, neither too small nor too large, between 0.2 mm and 1 mm. (c) The frequency of the flow reversal cycle shall be

SUBSTITUTE SHEET high and in all cases greater than 15 cycles per hour. d) The maximum temperature in the catalyst section must not be higher than 400 ° C. e) It is also essential that the metal wires which constitute the lining be placed in a position generally perpendicular to the air flows. Preferably, metal screens, cut in the form of circular disks, whose diameter is equal to that of the reactor, are used. As will be seen in the description of the experimental laws which follow and in the illustrative examples, when these conditions are satisfied, an apparatus is obtained which combines the following characteristics:

- Its volume is small - The initial period of time to heat it to the desired temperature is short

Its pressure loss as well as the cost of the fan and the acoustic level are low

- Its thermal efficiency is high. The definition of the thermal efficiency of the heat exchanger

Accumulator is the following:

Ts - Te

E = 1

Te - Te

Ts = Average air temperature at the outlet of the appliance

Te = mean temperature of the air at the inlet of the exchanger

Te = Average air temperature at the outlet of the catalyst section

Te is also the temperature of the air at the outlet of the fan. Ts is the average temperature of the exit air during a half cycle: for example, this temperature can rise from 20 ° C at the beginning of the half cycle to 30 ° C at the end of this half cycle. is the exit temperature of the catalyst and therefore, entry into the section of the cooling lining.

Te can be chosen close to 250 ° C for appliances referred to as "high temperature" and 80 ° C for those referred to as

SUBSTITUTE SHEET of "low temperature".

With an efficiency of 98% (E = 0.98), an inlet temperature of 20 ° C., a temperature of 220 ° C., the average temperature Ts would be equal to 24 ° C. The consumption of electrical energy is not only that necessary to compensate for this inefficiency because it is also necessary to take into account the consumption of the electric motor of the fan as well as the heat losses through insulation. Another fundamental characteristic of the apparatus according to the invention is that the pressure drop necessary to obtain this high efficiency of about 98% (between 95% and 99%) is still relatively high and of the same order of magnitude as that of the catalytic bed (6). It is known that catalysts based on palladium or silver may have a very high activity at relatively low temperatures: the operating temperature of the catalyst will always be chosen such that it allows to operate in the so-called external diffusion regime regime that is, it is not the catalytic activity that defines the efficiency of the catalyst bed but the transfer of material into the gas at the periphery of the catalyst particles.

Under these conditions, and as has been confirmed in our experiments, the pressure drop in a catalytic bed (6) which reduces by 90% the main pollutants (ozone, carbon monoxide, toluene, xylenes) is same order of magnitude as the sum of the head losses in the two packings (7) and (8). Therefore conventional commercial catalysts which are in the form of spheres or cylinders with a diameter of between 3 mm and 5 mm and can not use more expensive catalysts such as monoliths can be used.

But to take into account the very high efficiency of heat transfer obtained with high porosity packings, consisting of small diameter son and known disadvantages of commercial catalysts in which the particles of the catalytic metal are removed from the surface in contact with the gas flow, we studied,

ENT with success, the use of catalytic metallic wires as catalysts.

Thus, Palladium catalysts were obtained by electrochemical deposition on high volume porosity (90%) copper metal wire cloths. The yarns used in the manufacture of these fabrics preferably have a diameter of about 0.1 mm. As indicated by the illustrative examples, the efficiency of the depollution is very high.

In general, the thickness of the electrochemical deposit was 300 Angstroms.

It is obviously cheaper to perform an electrolytic coating of wires and fabrics than to deposit Palladium inside the pores of a support under conditions where the small particles obtained will not, subsequently, be susceptible to to sinter, that is, to reagglomerate.

A second series of tests was performed with an electrolytic deposit of silver on copper wires. As the illustrative example shows, the ozone removal efficiency is exceptional, especially when the ratio CO / 03 (carbon monoxide molecules to ozone molecules) in the air is greater than 10, which is always the case in ambient air, such as that treated in the apparatus according to the invention. It can be assumed that these pore-free catalysts work very well with this apparatus supplied with ambient air for the following four reasons: the quantities of phosphorus compounds, sulfur, zinc and silicon contained in the ambient air of the cities are very low

The device is equipped with a soil particle filter ides

- The surface of the accumulator wires is very large These accumulators are partially self-cleaning thanks to the cyclic inversion of the gas flow.

Poisoning of the catalyst is therefore relatively slow. We did not carry out sythematic tests of poisoning for very long periods of time.

OF SUBSTITUTION longε, but it can be admitted anyway that a catalyst consisting of son can be reactivated several times if necessary by heat treatment or washing with chemical solutions. In other words, just as it will be necessary to maintain the solid particle filter, it will also be necessary to maintain the catalyst.

EXAMPLE 1

APPARATUS FOR PURIFYING AIR FROM A

HOUSE FORCED CIRCULATION A cylindrical reactor 30 cm in diameter is used

(see Figure 1) wherein the section (6) containing the catalyst has a length of 5 cm and a total volume of 3.5 liters. The air flow to be treated is 200 m3 / hour measured at 20 ° C and at atmospheric pressure of Mexico City (590 mm of mercury).

With a rate of this order, we can maintain the clean atmosphere of a house or a surface apartment between 100 and 200 m2.

The catalyst (PC 163 type PROCATALYSE) consists of spherical particles of average diameter 0.32 cm. The density of these particles is such that the mass of the catalytic bed is 2.9 kg. At 220 ° C, with the above-mentioned airflow, the pressure drop across the catalytic bed is 3.5 cm - water, and the packing consists of a wire mesh woven from aluminum wires, approximately The wires have a diameter of 0.17 mm and the space between two wires is 1.6 mm Circular discs 30 cm in diameter are cut from the wire cloth and with these discs stacked, the accumulators 7 and 8. The length of each section is 15 cm Two diffusers (10) having a large number of small calibrated orifices make it possible to distribute the flow of air regularly over the entire section of the lining The porosity of the lining is 90% The weight of each section is 2.9 kg.

In normal operation, the pressure drop of each packing section is 7 cm - water to which it is necessary

REPLACEMENT add 1 cm - water ^" pressure drop of the diffuser.

As shown in Figure 1, the reactor is thermally insulated with a 5 cm thick layer of wool yarn (12). The resistor (9) can provide 250 watts. With the programmer (11) positioned to obtain a cycle of 1 minute (at each cycle, the two solenoid valves are reversed twice) the operation is started and the gradual increase in the temperature indicated by the thermocouple (13) located in the middle of the catalyst bed. After 25 minutes of operation, this temperature is maintained almost constant and equal to 220 ° C.

Under these conditions, the consumption of the fan motor is 250 watts and this energy is to be added to the 250 watts consumed by the resistor (9), which means that the air passing through the device undergoes an average temperature increase of about 9 degrees Celsius. This relatively warm air is quickly mixed with the colder air of the apartment or house; in any case a heat input of 500 watts does not allow to heat more than 1 or 2 ° C a house whose surface is between 100 and 200 M2 and we can say that the device works well as a purifier and not not as a heater. It is not possible to operate with such a low consumption of resistance (250 watts) while maintaining the catalyst at 220 ° C, as shown in the following comparative example if the cycle period is chosen larger.

This device can be called "high temperature" (220 ° C). Under these operating conditions (temperature, residence time of the air in the catalytic bed) the conversion of ozone is very large (the concentration decreases from 1 ppmv (parts per million by volume) to less than 0.05 ppmv ) as well as carbon monoxide (from 5 ppmv to o.5 ppmv) and a typical reactive hydrocarbon (from 1 ppmv to less than 0.05 ppmv in the case of toluene). The efficiency of this industrial catalyst is therefore consistent with that indicated by its manufacturer and this is not the purpose of this

SUBSTITUTE SHEET invention.

EXAMPLE 2

The programmer of the apparatus of Example 1 is set at 6 cycles per hour. To maintain the temperature at 220 ° C, it is necessary, through the use of a transformer, to increase the power consumed in the resistance up to 450 watts.

EXAMPLE 3

This comparative example shows another advantage of the operation with short cycles. With 6 cycles per hour, as in Example 2, the catalyst can be maintained at

220 ° C with the same power consumed (250 watts) as in Example 1 but provided to change the length of the cooling packings and preheating. By increasing the length of each lining to 25 cm, we can operate well with 250 watts but the pressure drop of each lining is multiplied by 1.7 (12 cm-water) and we can no longer use a low pressure fan and low acoustic level. Another solution would be not to increase the length of the packings, but the diameter of the device then becomes larger and more expensive.

EXAMPLE 4

This comparative example illustrates the advantage associated with the use of highly porous metal fabrics. By using 50 mesh aluminum wire cloths, made of 0.2 mm wires, porosity 0.67 and reducing the length of the packings to 6 cm, the power needed to maintain the catalyst at 220 ° C is still equal to 250 watts (same efficiency as in Example 1). But the pressure drop of each lining is equal to 12 cm-water.

It is obvious that the pressure drop would increase by a factor of (12/7) ² = 3 to reach 21 cm if we simultaneously combine the two modifications of Examples 3 and 4, which clearly shows one of the advantages of the apparatus according to The invention.

EXAMPLE 5

This comparative example illustrates the advantage associated with the use of good conductor metal wire cloths

REPLACEMENT ISLAND of heat, by comparing the performance of fabrics made of less conductive yarns such as glass fibers.

By keeping all the values of the design and operating parameters identical to those of Example 1, the aluminum fabrics are replaced by fiberglass fabrics having similar geometrical characteristics. It is observed that it is necessary to bring the power consumed to the resistance at 420 watts to maintain the catalyst temperature at 220 ° C.

Heat exchange is less efficient: efficiency goes from 98% to 95.8%. Since the specific densities and heats of aluminum and glass are approximately equal, but the thermal conductivities of the two materials are very different (that of aluminum is 200 times greater than that of glass), this result shows that the radial conduction of the heat in the same wafer pad contributes to increase the efficiency of the lining and to reduce the effect of poor distribution of air at the entrance of the lining.

For this reason, any lining not consisting of metal wires oriented perpendicular to the axis of the apparatus (ie the direction of the gas flow) will be less effective. This would be particularly the case with metallic or ceramic particles for which the radial conduction is weak and due solely to the points of contact of the particles with each other.

EXAMPLE 6

The length of the packings of Example 1 is reduced by 15 to 8 cm and the power consumed by the resistor (125 watts) is also reduced by half, which results in a catalyst temperature equal to 70 ° C. The pressure drop in each lining is 3.7 cm and in the catalyst bed 2.6 cm. The power consumed by the engine is 150 watts.

This system corresponds to a "low temperature" operation (70 ° C). It can be noted that energy consumption is almost halved. As shown the chemical analyzes, the conversion of ozone remains high (about 85%) but conversions of carbon monoxide and toluene are low (less than 10%).

Before presenting the other examples, we present some comments on the previous examples.

The two important conclusions we draw from the previous examples are:

1) Regardless of the temperature of the catalyst, it is always possible to operate with a thermal efficiency of the order of 97%, that is to say that the temperature of the catalyst can be increased by 50.degree. the power consumed in the resistance that goes from 250 watts to 315 watts. But an increase of 50 ° C. greatly increases the intrinsic activity of the catalyst, which in the case of palladium, the preferred catalyst according to the invention, can be multiplied by a factor of 6.

Therefore it is always possible to operate in external diffusion mode that is to say with diffusional braking at the periphery of the catalyst particles without the consumption of electrical energy is greatly increased.

2) Another conclusion is that to obtain a thermal efficiency greater than 95% the pressure drop is generally greater in the packings than in the catalytic bed. This conclusion remains valid even for the highly efficient system and low pressure drop of the apparatus according to the invention (high porosity, conducting fabrics placed perpendicularly to the gas flow). This means that a catalyst of low pressure drop but high price such as the monolith is not essential and spherical particles of porous bead can be used.

The combination of these two conclusions leads us to use as preferred catalysts according to the invention, a catalyst without internal porosity (without pores) constituted by metal son with the catalytic metal deposited electrochemically on the surface of these son. The catalytic packing is in every respect similar to the packing of heat recovery and its efficiency, that is to say, its catalytic efficiency defined in this caε as the ratio of the material transfer to the pressure drop will be very high if the porosity of the catalytic packing is large (greater than 70% ). Reducing the size of the wires (diameter less than 0.5 mm) also makes it possible to reduce the amount of support metal. But it is obvious that the weight of the support metal or the pressure drop of the fiber bed or fabrics are not determining factors in the investment cost or the operating cost of the device.

The main gain that results from the use of these electrochemically coated catalysts is the low cost of the manufacturing process and the fact that they are thermally stable without the additives that are used with the porous catalysts in order to avoid sintering. metal particles deposited in the pores.

EXAMPLE 7

"Small diameter apparatus for the study of different catalysts". A cylindrical reactor 3 cm in diameter identical to the reactor shown in FIG. 1 is used. The only difference between this reactor and the reactor of example 1 is the diameter of the reactor, the pipes and the fan.

With a flow rate of 2 m 3 / hour and all the other identical operating parameters, it is observed that the thermal efficiency of the accumulator-generator as well as the catalytic efficiency of the bed of spherical particles are identical to those of Example 1. In other words, the diameter of the reactor has no effect on the yields, which was predictable, given the relatively large pressure drop of the packings and the radial heat conduction due to aluminum son.

By using these same aluminum foils as a palladium support which is deposited electrochemically in the form of a continuous layer of 300 angstroms (30 gauge) of thickness, it is observed that with a catalytic bed of 10 cm in length, pollution control efficiency (ozone, carbon monoxide, toluene) is significantly higher than that of Example 1, while the pressure drop in the catalytic bed is 5 cm water, that is to say less than the loss of charge deε garniεεageε the accumulator. EXAMPLE 8 The aluminum foils used as support in Example 7 are replaced by copper cloths and electrochemically coated under the same conditions as those of Example 7.

This new catalyst, in a preliminary operation is conditioned at 600 ° C for 14 hours in a mixture of 1% methane, 4% oxygen and 95% nitrogen (see method of Briot and Primet, Applied Catalysis, 68 (1991) p. 301-314).

Using this new catalyst, under the conditions of Example 7 but at a lower temperature, 180 ° C., it has been found that the depollution efficiency is essentially identical but the energy consumption in the resistor is reduced by 250 watts. at 200 watts (energy required to maintain the catalyst at 180 ° C.) EXAMPLE 9 With the aluminum foils of Example 7, using a silver coating (thickness: 300 Angstroms), the air is treated. contaminated with 1 ppmv of ozone and 10 ppmv of carbon monoxide The catalyst temperature is maintained at 90 ° C using the electrical transformer The conversion of ozone is greater than 90% but the carbon monoxide n is little transformed.

This low-temperature system, based on either palladium or silver, should be able to treat large flows of polluted air at a very low cost, by optimizing the total electrical consumption (in the resistor and in the engine).

These large diameter devices (2 m in diameter or more) could be used in the depollution of large buildings, recreation areas or sports etc. The patent applicant confirms that the best method he currently knows to put in practice, the invention that is the subject of this deposit is that which results from the preceding description.

NT

Claims

1 - Apparatus for purifying air which comprises a catalytic system for reducing the content of pollutant compounds present in the air, intended for use in offices, housing, motor cars, playgrounds, polluted streets etc ..., characterized in that it combines a fan equipped with a filter that supplies the air to be purified in a piping system equipped with two or more solenoid valves operated by a cyclic programmer; the solenoid valves are actuated so as to perform the periodic inversion of the air flow that passes through the reactor tube; this tube contains inside it a palladium or silver catalyst and comprises, on either side of the catalytic packing, two packings constituted by non-catalytic solids; in the catalyst bed is placed the electrical resistance whose role is to heat the air as it passes through the catalytic packing; at the ends of the non-catalytic packings, two diffusers consisting of plates pierced with numerous calibrated perforations have been placed, which make it possible to regularly distribute the air flow over the entire section of the packings; the wall of the tube is insulated by a layer of glass fibers.

2 - Apparatus according to claim 1, characterized in that the non-catalytic packings are constituted by circular discs of wire cloth, stacked and arranged perpendicularly to the direction of the air flow.

3 - Apparatus according to claims 1 and 2, characterized in that the non-catalytic packings consist of wire meshes whose threads have a diameter less than 1 mm and preferably included in the range 0.2 mm to 0.5 mm and also that the volume porosity of these fillings is included between 70 and 90%.

4 - Apparatus according to claimsε 1 to 3, characterized in that the preferred metal fabric according to the invention consists of aluminum son, stainless metal of high thermal conductivity and high specific heat.

SUBSTITUTE SHEET 5 - Apparatus according to claims 1 to 4, characterized in that the periodic everthing of the direction of the gas flow is designed to perform very frequent inversiones, up to 200 inversions per hour and, in all cases, more than 20 inversiones per hour.

6 - Apparatus according to claims 1 to 5, characterized in that the catalyst consists of porous spherical or cylindrical particles containing palladium or silver deposits in the pores. 7 - Apparatus according to claims 1 to 5, characterized in that the catalyst consists of electrochemically coated metal foils palladium or silver.

SUBSTITUTE SHEET